Transducer for bioacoustic signals

ABSTRACT

In well-known electronic stethoscopic transducers the sensitive element is influenced by signals transmitted via the skin, and the rear side is enclosed in a housing to prevent airborne noise from reaching the sensitive element. According to the invention, an improved signal-to-noise ratio is obtained by letting the transducer be a piezoelectric transflexural diaphragm in contact with the skin, the rear side of the diaphragm communicating with the surronding air via an acoustical network, thereby receiving airborne noise which acts to counteract the influence of noise coming from the skin.

BACKGROUND OF THE INVENTION

1. Field of Invention

The invention concerns a transducer for bioacoustic signals comprising atransducer element having a front side and a rear side, the front sideof which may establish an intimate contact with the surface of a bodypart, said surface being the transmitter of direct interior sound fromthe body, said transducer element being mounted in a housing subject toairborne noise, and having a surface surrounding the front side of saidtransducing element, said element and said surrounding surface being inintimate contact with the surface of said body part during use.

2. Description of Related Art

Transducers for bioacoustic signals emanating from a body usually belongto two main types applied to an outside surface of the body. One type isa microphone in principle, in which the vibration of a delimited area ofskin is picked up as pressure variations in the air surrounding the areaof skin, usually the pressure variations in a closed volume delimited bythe skin, the microphone diaphragm, and the housing. An enclosed volumeis essential to obtain a good low frequency response as well asprotection from extraneous airborne noise—one early example is thestandard binaural stethoscope in which the bell defines the volume. Thesecond type is an accelerometer in principle, in which a light-weighthousing rests against the part of the body and the inertial mass insideprovides reference in the generation of signals proportional to theinstant acceleration. This type has in itself a good protection againstextraneous airborne noises, but the sensitivity decreases and theelectrical noise increases very much in the lower end of the frequencyrange of interest, unless the inertial mass is increased to a value inwhich it unavoidably influences the actual measurement. There is hence aneed for an improved transducer.

It has been determined that extraneous airborne noise in general entersthe transducer mainly by two routes. One is direct airborne influence onthe transducing element itself, e.g., a microphone diaphragm. The otheris by means of influence on the diaphragm from the skin in touch withthe diaphragm, while the housing is receiving airborne noise. A furthercontribution may be airborne noise radiated into the body around thehousing surrounding the transducer in contact with the body, saidairborne noise being converted to pressure waves which are re-radiatedfrom the part of the skin directly in touch with the diaphragm. Thistype of noise injection is not avoided by enclosing the area of skin,however the phase relations to the desired signal are such that thecontribution would generally be of minor importance. There is hence afurther need to address in an improved construction of a transducer forbioacoustic signals.

In U.S. Pat. No. 5,610,987 a solution is given, which utilises apiezoelectric transflexural diaphragm in direct contact with the skin inthe area within the surrounding housing. In this case, the noise signalis coupled to the diaphragm without re-radiation, and the rear of thediaphragm is shielded against extraneous noises by the housing. In orderto obtain noise cancellation, this patent also describes that thehousing contains an identical but outwards-facing piezoelectrictransflexural diaphragm which is only subjected to airborne noise, andthat a further identical transducer is placed in contact with the bodysome distance from the first transducer. Extensive digital signalprocessing enables a high degree of elimination of the undesired noises.This makes the equipment expensive and causes a need for re-programmingif the sensor part is exchanged.

U.S. Pat. No. 6,028,942 relates to the chestpiece of an acoustic,non-amplified stethoscope having noise balancing means, in that aresonator is coupled to the reverse side of the diaphragm. An embodimentusing amplification is also shown. The purpose is to compensate thenoise that is radiated into the tissue surrounding the front end whenapplied to the skin and which is added to the desired signal. There isan air space between the tissue and the diaphragm that provides theoutput signal, however this remarkably reduces the usefulness of thedevice in practice. It is probable that the elaborate equivalentcircuits used in U.S. Pat. No. 6,028,942 are misleading, because they donot take into account the pickup of airborne noise via the transducerhousing mass itself.

SUMMARY OF THE INVENTION

The invention is based on a recognition that there is indeed asignificant pickup of airborne noise by the housing of the transducer,and that the ensuing vibration of the housing acts on the diaphragm bypressing against the outer surface of the body. If the reverse side ofthe diaphragm picks up the extraneous airborne noise in a suitable phaserelationship, the influence of the airborne noise, will be effectivelyeliminated in a frequency interval of interest. The proper phaserelationship may be available in a very narrow frequency range by justproviding access for airborne noise to the rear side of the diaphragm,but further improvement in a frequency interval of interest may beobtained by suitable acoustical loading of the rear side of thediaphragm. In particular, it has been demonstrated that an importantimprovement is obtained by reducing that proportion of the surface ofthe transducer presented to the outer surface of the body that isconstituted by the diaphragm. Hence, the invention is in particular thatthe effective area of the transducing element is less than 50% of thearea of the surrounding surface of the housing and in that the rear sideof the transducing element is loaded by an acoustical network which isin communication with the surrounding air, said loading creating anextinguishing relationship between airborne noise signals influencingthe front and rear sides of the transducing element respectively. Theinvention takes account of the fact that there are several paths of boththe desired physiological signals and the offending airborne noises, andthat influencing the housing also causes an influence on the transducingelement.

An advantageous embodiment of the invention is in particular that theeffective area of the transducing element is between ½ and 1/1000 of thearea of the surrounding surface of the housing. It has been determinedthat there is an improvement in performance when the respective areasare held within these proportions. The effect may be related to the areaof contact to the skin and the density of the underlying tissue. Byeffective area is meant the area of the diaphragm that is actuallyflexing and contributing to the output, i.e. it is usually less than theopening in the surrounding surface.

In a further advantageous modification this ratio is within the interval0.2≧ad/ah≧0.05.

An advantageous embodiment is in particular that the transducing elementis a compound diaphragm giving an electrical output when exposed tobending. This may be obtained in the form of what has been termed apiezoelectric transflexural diaphragm, which is in fact a very thinpiezoelectric layer, one side of which is usually bonded to a metaldiaphragm and which has a metal layer deposited on the other side. Thislaminate reacts to shear stresses in the piezoelectric layer occurringwhen the diaphragm is bent inwards and outwards by generating a voltagedifference between the metal diaphragm and the metal deposit.

A further advantageous embodiment is in particular that the transducingelement is a compound diaphragm giving an electrical output when exposedto differential stretching of the front side with respect to the rearside of the diaphragm. This is slightly different construction, whichmay give advantages for particular ad/ah ratios.

A further advantageous embodiment is in particular that the acousticalnetwork consists of a cavity in the housing being indirectly influencedby airborne noise.

A further advantageous embodiment is in particular that the acousticalnetwork consists of a cavity and at least one port in the housing. Thisis in fact an enclosure for the diaphragm with a leak, and by suitablyplacing the resonant frequency of this cavity volume and portcombination, an extension of the frequency response and in particular ofthe range of noise suppression may be obtained.

A further advantageous embodiment is in particular that the acousticalnetwork consists of a cylindrical conduit having essentially the samediameter as the diaphragm. This corresponds to letting the diaphragm sitin the bottom of a well, which provides a good shielding and mechanicalprotection of the diaphragm and connections and reducing the risk thatthe closure of a port will change the frequency response of thetransducer.

A further advantageous embodiment is in particular that the port isconstituted by a narrow slit. This has the particular advantage that itis difficult accidentally to cover the whole length of the slit, whichreduces the risk that the port will change its properties materiallyduring practical use. A further embodiment provides a non-wettablematerial for the slit surround.

An advantageous embodiment of the invention is in particular that anelastic material capable of transmitting mechanical vibration isprovided in sealing relationship between the skin and the diaphragm.While the diaphragm may be made of stainless steel which is generallyregarded as inert with respect to skin, there may be cases of nickelallergy, and for this reason and for normal surface protection of thediaphragm it may be desirable to provide the transducer with a layer ofan elastomer. The skilled person will be able to select a material whichhas suitable transmission properties for this application.

A further advantageous embodiment is in particular that the acousticalnetwork means comprises damping material.

A further advantageous embodiment is in particular that the cylindricalconduit is provided with a damping material.

A further advantageous embodiment is in particular that damping materialis used as a resistive element in a port.

A further advantageous embodiment is in particular that the dampingmaterial has water-repellent qualities.

The invention will be further described with reference to the drawing,in which three transducers are described, the first (Type I) having anenclosed space in contact with the rear of the diaphragm and only inindirect contact with the surrounding air, the second (Type II) havingan opening leading to the rear of the diaphragm as the most primitivecase of an acoustical network directly connected to the surrounding airthat will function according to the invention, and the third (Type III)having a closed volume with a port as a further but more sophisticatedcase of an acoustical network that will function according to theinvention. Each transducer is documented by figures showing itsequivalent diagram and figures showing the results obtained bysimulation based on the dimensions of a useful transducer and the forcescreated during its practical use. Note that the absolute levels of thecurves expressed in dB have no physical meaning as such, as the soundsources in the various situations have been normalised to unity. Thecontent of the figures is as follows:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a transducer according to prior art placed on the skin of abody,

FIG. 1 a shows the construction of a prior art transducer,

FIG. 2 identifies the components of a transducer according to a firstembodiment of the invention (Type I),

FIG. 3 shows an electrical equivalent circuit of the transducer shown inFIG. 2,

FIG. 4 shows the S/N performance of a transducer shown in FIG. 2,

FIG. 5 identifies the components of a transducer according to a secondembodiment of the invention (Type II),

FIG. 6 shows the S/N performance of a transducer shown in FIG. 7,

FIG. 7 shows the construction of a transducer according to a thirdembodiment of the invention (Type III),

FIG. 8 identifies the components of the transducer shown in FIG. 7,

FIG. 9 shows an electrical equivalent circuit of the transducer shown iFIG. 7,

FIG. 10 shows the S/N ratio performance of the Type III transducer,

FIG. 11 shows results of changes of area ratio according to one aspectof the invention, changing the area of the housing,

FIG. 12 shows results of changes of area ratio according to one aspectof the invention, changing the area of the diaphragm, and

FIG. 13 shows an equivalent circuit for determining the sensitivity tothe desired physiological signals.

DETAILED DESCRIPTION OF THE INVENTION

In the equivalent circuits and the expressions of their properties, thefollowing nomenclature is adhered to:

-   -   Sh: Effective application area towards tissue of inactive        transducer housing (e.g. calculated from the housing radius ah        and the sensor diaphragm radius ad for concentrically        distributed area elements)    -   Zhr: Mechanical domain radiation impedance from transducer        housing into the ambience calculated from Sh    -   Zhm: Mechanical domain impedance of the transducer housing mass        (Mh)    -   Zht: Mechanical domain tissue impedance acting on transducer        housing (calculated from Sh). Consists of discrete elements Rht,        Mht and Cht    -   Zha: Mechanical impedance of hand/arm holding the transducer        housing (approximately 40 Ns/m)    -   Sd: Effective application area towards tissue of active sensor        diaphragm (calculated from the sensor diaphragm radius ad for        concentrically distributed area elements)    -   Zhr: Mechanical domain radiation impedance from sensor diaphragm        (simple rear side transducer opening of same area as diaphragm)        into the ambience calculated from Sd    -   Zdc: Mechanical domain impedance of sensor diaphragm compliance        (Cd)    -   Zdt: Mechanical domain tissue impedance acting on sensor        diaphragm (calculated from Sd). Consists of discrete elements        Rdt, Mdt and Cdt    -   Zpv: Acoustical domain impedance of air-chamber volume (Vp)        compliance (behind sensor diaphragm, not including port)    -   Zpp: Acoustical domain impedance of port consisting of the        discrete elements Rp and Mp    -   Zpr: Mechanical domain radiation impedance from port opening        into the ambience

The calculations are based on the following design values of theproperties of a housing with transducer according to one embodiment ofthe invention:

-   -   ah=20 mm    -   ad=6 mm    -   mh=50 g    -   cd=8 m/N    -   Vp=0.4ml    -   a=0.5 mm    -   b=4mm    -   l=15mm    -   F=4 N (the force of application of the transducer on the thorax)

In FIG. 1 is seen a section of a body resting on its back with atransducer T placed against the skin. The transducer shown in FIG. 1 ais comprised of an outer housing 4 having an inner housing 3 holding adiaphragm 1 by its rim and creating a surround 5. Furthermore, there isa clamping arrangement 6 for the signal lead and its electrostaticshielding. The housing may also hold a pre-amplifier and impedanceconverter 2, e.g. using a phantom power supply. The diaphragm 1 may be atransflexural piezoelectric laminate known per se which gives off avoltage when flexed or a piezoelectric element P. One electrode consistsof the actual metallic diaphragm, and the other is deposited onto theother side of the thin sheet of piezoelectric material. The diaphragm ismounted flush with or at least in the same plane as the surrounding partof the housing, and the surround 5 has a diameter or width such thatairtight contact with the skin ensured. The housing is closed, therebyshielding the rear side of the diaphragm from airborne sound andcreating a cavity C, which is a general representation of the prior art.

In FIG. 2 is seen a simplified layout of the components in a transduceraccording to the invention, and in FIG. 3 is shown the electricalequivalent circuit of the transmission path from ambient noise via thetransducer housing and to the front side of the sensor diaphragm (theside in touch with the body). Ambient noise is introduced to the frontside of the sensor diaphragm (facing the tissue) as the ambient noisepressure signal pushes on the transducer housing, thereby causingcompression (pressure) in the underlying tissue which acts on the sensordiaphragm. A rigid mass-less piston (of surface area Sd) supported by aspring (Zd) attached to the transducer housing is a valid approximationfor the fixation of the flexible sensor diaphragm onto the transducerhousing.

For reasons of convenience the ambient noise picked-up can be split intwo ‘stages’, first the ambient noise pressure signal couples to thetransducer housing (via the housing radiation impedance acting asgenerator output impedance) where it may be transformed to a mechanicalforce signal and the loading from housing mass as well as attachedtissue impedance (e.g., thorax impedance) may be introduced. Then thisresulting input force signal undergoes an area transformation, from theinactive housing application area Sh over to the active sensor area Sd,where the loading contributions from the sensor diaphragm (primarilymechanical compliance) and its underlying tissue can be applied. FIG. 3shows the electrical equivalent circuit of the transmission path fromambient noise via the transducer housing and to the front side of thesensor diaphragm. The sensor output is assumed proportional with theforce across the sensor diaphragm compliance element. The resultingforce acting on the sensor diaphragm may be calculated using (1). Notethe sign inversion on the final impact due to the reaction from thetissue causing a downward force on top of the housing to act upwards onthe sensor diaphragm. $\begin{matrix}{F_{{amb},{closed}} = {{- P_{a}}{Sd}\frac{\left( \frac{Sh}{Sd} \right)^{2}\left( {{Zd}\left. {Zdt} \right)} \right.}{\left( \frac{Sh}{Sd} \right)^{2}\left( {{{Zd}\left. {Zdt} \right)} + {Zhr} + {Zht} + {Zhm}} \right.}}} & (1)\end{matrix}$

In the expression Pa denotes the input ambient pressure noise signal andFamb, closed denotes the resulting impact on the frontal side of thesensor diaphragm. Furthermore Zd is the sensor diaphragm mechanicalimpedance calculated from (2) where Cd is the diaphragm mechanicalcompliance $\begin{matrix}{{Zd} = \frac{1}{j\quad\omega\quad{Cd}}} & (2)\end{matrix}$

Furthermore Zdt denotes contribution from the tissue impedance acting onthe sensor diaphragm, a single degree of freedom system (SDOF mass-,compliance- and damping in series) in dependence of application forceand application surface area e.g. as adapted from Vermarien H. and vanVollenhoven E.: “The recording of heart vibrations: a problem ofvibration measurement on soft tissue”, Medical & Biological Engineering& Computing, 1984, 22, pp 168-178. In accordance with this source theaverage human thorax tissue impedance associated with a circularapplication surface of diameter 30 mm applied under 0.6 N of forceagainst the tissue would result in an approximate set of SDOF elementsof mass Mt=5.4 gr., compliance Ct=0.62 mm/N and damping Rt=4.8 Ns/m. Thetotal tissue impedance would add up to $\begin{matrix}{{Zt} = {{Rt} + {j\quad\omega\quad{Mt}}\quad + {\frac{1}{{j\omega}\quad{Ct}}\left\lbrack {{Ns}\text{/}m} \right\rbrack}}} & (3)\end{matrix}$

The housing mass mechanical loading impedance Zhm may be calculatedusing (4) where Mh is the housing mass.Zhm=jωMh  (4)

The radiation impedance Zhr may be estimated from (5) which calculatesthe impedance out into a 2π-space, in this equation α_(h) is theequivalent radius of a circular rigid piston of same area as the housingradiation area (e.g. Sh) and k is the wave number$\left( {k = \frac{\omega}{c}} \right).$ $\begin{matrix}{{Zhr} = {{\frac{\pi}{2}\rho\quad{ck}^{2}\alpha_{h}^{4}} + {{j\omega}\frac{8}{3}{\rho\alpha}_{h}^{3}}}} & (5)\end{matrix}$

In the case of the user holding the transducer by hand the hand/armimpedance loading may be included in series connection with Zhr, Zht andZhm working within the Sh area domain. The effect on (1) is an added Zhaimpedance element in the denominator, whereZha≈40[Ns/m]  (6)

In the most practical situations the rear side of the sensor diaphragmfaces an enclosed volume (room allowing for diaphragm deflection) andthe inherent loading of this element will in principle also affect thesensor diaphragm deflection. However, typically, this enclosed spacewill act as a soft spring compared to the sensor diaphragm and hencehave no practical importance and as a consequence the above equivalentcircuit does not contain this element. If otherwise required arepresentative air-chamber volume compliance impedance element should beinserted in the model in series with Zdt.

The usefulness of a transducer for physiological signals depends to alarge degree not only on its ability to suppress the influence of noise,but equally on its ability to receive the relevant physiologicalsignals. The input to the transducer occurs via two paths, one beingacross the thorax impedance, the other being via the housing. The soundsource itself is regarded as a high-impedance velocity sound source, andhence the electrical equivalent of the sound transmission forphysiological signals may be determined according to the structure shownin FIG. 13, using the nomenclature defined above. The influence fromhand/arm holding the transducer housing may be relevant in somesituations, and the loading contribution from Zha may then beimplemented by applying it in series with Mht and Mh as shown by the xon the drawing. Furthermore an inclusion of an enclosed air-cavityvolume is implemented by adding this loading contribution in series withCd.

The force acting on the sensor diaphragm may be calculated from themodel in accordance with (7) $\begin{matrix}{F_{{phys}\quad} = {V_{t}\frac{Zd}{{Zd} + {Zmdt}}\frac{\left( {{Zmht} + {Zmh}} \right)\left\lbrack {\left( {{Zcdt} + {Zrdt}} \right)\left. \left( {{Zmdt} + {Zd}} \right) \right\rbrack} \right.}{{Zmht} + {Zmh} + {Zcht} + {Zrht} + {\left( {{Zcdt} + {Zrdt}} \right){\left( {{Zmdt} + {Zd}} \right)}}}}} & (7)\end{matrix}$

With expressions of the sensitivities of the transducer towards both thedesired and the undesired signals being available, the performance of atransducer may usefully be expressed as the signal-to-noise (S/N) ratio,and it is frequency dependent. In order to compare examples of technicalsolutions or embodiments the S/N ratio will be given as a function offrequency for some typical configurations. As mentioned above, thevalues in dB are relative only.

A transducer of Type I and with the dimensions and weight given abovewill perform as shown in FIG. 4. Here the variation has been given inthe parameters ah (top) and ad (bottom). Solid lines indicate thenominal value, the dashed lines indicate double the nominal respectivevalues and the dotted lines indicate half the nominal respective values.From observing the results from the parameter variation it becomes clearthat an increased ratio between inactive and active transducerapplication area improves the overall suppression of ambient noise. Theresult from decreasing the active area while holding the inactive fixed(ad variation) most clearly demonstrates the effect. However also forthe ah-variation an increased inactive area (with fixed active sensorarea) tends to move the resonant notch upward in frequency, therebyeffectively expanding the operating frequency range of the transducersystem. Typical auscultation sound information lies below the 1000 Hzlimit and by pushing the resonance notch above this point, whilemaintaining a high level of suppression just beneath it, effectivelyimproves the practical signal-to-noise ratio more, in comparison totuning the resonance point lower and trying to compensate with evenbetter ambient noise suppression further above the resonance point.

Due to the governing idea of the importance of the ratio between active-and inactive transducer application area (18) more than the absolutevalue of the physical radii of these surfaces themselves, the effectivevariation for the radii are defined as those causing halving, unity ordoubling of the area ratio Sd/Sh from its nominal value. For thecircular and concentrically distributed area elements the area ratio maybe expressed as $\begin{matrix}{\frac{Sd}{Sh} = \frac{a_{d}^{2}}{a_{h}^{2} - a_{d}^{2}}} & (8)\end{matrix}$

From observing the results of the parameter analysis on the closedtransducer system (FIG. 4) it becomes clear that an increased ratiobetween inactive and active transducer application area improves theoverall suppression of ambient noise. The result from decreasing theactive area while holding the inactive fixed (ad variation) most clearlydemonstrates the effect. However also for the ah-variation an increasedinactive area (with fixed active sensor area) tends to move the resonantnotch upward infrequency, thereby effectively expanding the operatingfrequency range of the transducer system. Typical auscultation soundinformation lies below the 1000 Hz limit and by pushing the resonancenotch above this point, while maintaining a high level of suppressionjust beneath it, effectively improves the practical signal-to-noiseratio more compared to tuning the resonance point lower and trying tocompensate with even better ambient noise suppression further above theresonance point.

In order to reduce the susceptibility towards air-borne ambient noise ofthe transducer without however significantly degrading its sensitivitytowards physiological vibration signals the concept of opening thetransducer housing behind the sensor element has been tried (Type II),thereby allowing for counteracting ambient noise to enter the system.The simplest kind of rear side sound passage is a wide opening, causingthe resulting effective pressure on the diaphragm rear side to equalthat of the pressure acting on the transducer housing. FIG. 5 shows thephysical layout of the simple opened transducer system with the simpleopening consisting of a cylindrical conduit having essentially the samediameter as the sensor diaphragm. Thereby the ambient noise is allowedto reach the rear side of the diaphragm without any filtering action.

The effect of the simple opening on the total system ambient noiseresponse may be calculated by the according to (9) and (10), whichsimply subtracts the resulting rear side force component from thecomplementary frontal side force component as provided above.$\begin{matrix}{F_{d,{rear},{simple}} = {P_{a}{Sd}}} & (9) \\{F_{{amb},{simple}} = {{F_{d,{closed}} + F_{d,{rear},{simple}}} = {P_{a}{Sd}\frac{{Zhr} + {Zht} + {Zhm}}{\left( \frac{Sh}{Sd} \right)^{2}\left( {{{Zd}\left. {Zdt} \right)} + {Zhr} + {Zht} + {Zhm}} \right.}}}} & (10)\end{matrix}$

An interesting detail that can be deduced from (10) is that an increasedin-active transducer housing application area Sh as well as a reducedtransducer housing mass seems to effectively reduce the transducer'ssusceptibility towards ambient noise.

A transducer of Type II and with the dimensions and weight given abovewill perform as shown in FIG. 6. Here the variation has been given inthe parameters ah (top) and ad (bottom). Solid lines indicate thenominal value, the dashed lines indicate double the nominal respectivevalues and the dotted lines indicate half the nominal respective values.

Inspection of the results from the simulation of the simple openedtransducer system performance (FIG. 6) leaves the impression that manyof the same features as stated for Type I are active in this situationas well. The fundamental difference however, of the high frequencyresponse (at some point) always becoming inferior to that of the closedsystem is evident for every situation. The only way to handle thisnegative effect is to push the high frequency resonance notch up high aspossible in the frequency range, and this feature is very convincinglydealt with by reducing the Sd/Sh area ratio. Again, the reduction of adis identified as having the most powerful impact on the transducerperformance.

This type of transducer may advantageously be provided with acousticresistance means in the large opening connecting the rear side of thediaphragm to the surrounding air, and preferably flush with the outersurface of the housing. This acoustic resistance means will contributeto an improved S/N ratio in the relatively higher frequency range of thetransducer. The means is advantageously chosen from the group comprisingfelt and non-woven fibrous materials and preferably provided with awater-repellent outer surface. This has the double function of providingnot only a well-defined resistive part of the impedance predominantlyactive in the higher frequency range, but it also provides environmentalprotection from dust and humidity for the sensitive diaphragm.Furthermore, this type of protection will not change its acousticalproperties, even when subjected to dust or water spray in limitedquantities.

For both types I and II it has been demonstrated how the ratio betweenactive and inactive application surface area Sd/Sh (more than theindividual sizes of these areas seen isolated) is a central element inthe optimization of the transducer system to maximal ambient noisereduction. In the nominal parameter values the area ratio had a value of˜1/10 and during the variation of the radii the value ˜1/20 was testedand proven superior to the nominal value.

FIGS. 11 and 12 show simulations for the area ratio values 1/20, 1/10and ½, realized either through ah variation (FIG. 11) or ad variation(FIG. 12). Each graph shows the noise performance for the closed systemand furthermore the improvement of the simple opened system over theclosed system. The lower set of curves represents the closed transducersystem (Type I) response and upper set of curves show the improvement ofthe simple opened system (Type II) over the closed system. Area ratioSd/Sh values of 1/20 (dashed), 1/10 (solid) and 1/2 (dotted) have beenshown.

The closed system response changes dramatically under the influence ofthe area ratio alteration, both in overall level as well as the positionof its resonance changes from ˜1200 Hz (with Sd/Sh=1/20) down to ˜200 Hz(with Sd/Sh=1/2). Looking at the relative performance of the simpleopened system it is seen, that above the closed system responseresonance the simple opened system is inferior to the closed. For thesimple opened system the area ratio Sd/Sh=1/2 defines a lower limit ofbeneficial value of the rear side in-coupling to be located at approx.200 Hz (for these defined physical conditions).

Considering the frequency range up to the above 200 Hz in isolation thisis of cause a valid improvement in that specific frequency range, and itwould possibly be satisfactory if the interest solely covered e.g.fundamental heart sounds and low frequency murmurs. To be used in a moregeneral auscultation system (stethoscopes etc.) the 200 Hz limit is notsatisfactory at all, as most hard-to-hear heart murmurs (as well asdelicate lung sounds) all reside in the frequency range above 200 Hz. Inthe other end of the scale the Sd/Sh=1/20 performs very well indeed asit effectively moves the closed system resonance above 1000 Hz, therebyallowing for good damping in a very wide operating frequency range.

FIG. 9 shows the physical layout of a transducer system (Type III)having a combined port (an acoustical vent having a resistance and amass element) and air-cavity volume performing a second order low-passfiltering of the ambient noise before it meets the sensor diaphragm rearside. FIG. 7 shows a Type III transducer in greater detail fitted in ahousing. The cavity 7 is in communication with the surrounding air bymeans of a port 8 with well-defined properties. The surface of thediaphragm touching the skin may be protected by a coat or layer ofmaterial 9 that will not influence the pickup by the diaphragm, i.e. itshould posses properties similar to the tissue that the diaphragm istouching. This is indicated by the hatching of the slight depressionformed in the surround 5 in which the diaphragm is placed. It will beseen in the figure that the radius ad of the diaphragm is ca. 50% of theradius ah of the housing, corresponding to an area proportion of ca.25%.

The port/volume system is characteristic in its resonance frequency andits overshoot at resonance, below resonance the system is to beconsidered approximately as a simple opening whereas the response aboveresonance is a second order low-pass roll-off. In order to accuratelyestimate the response, the contribution from the sensor diaphragmcompliance possibly needs to be included in the modeling. The diaphragmcompliance will act in parallel with the air-cavity volume complianceand in cases where the volume compliance is not significantly larger thediaphragm compliance will induce a reduced resonance frequency for thecomplete system. The equivalent circuit is shown in FIG. 8, whichessentially shows the transmission path from the ambient noise floor tothe rear side of the sensor diaphragm.

From the model the contribution from the rear side ambient noisepressure signal input may be calculated using (11) $\begin{matrix}{F_{d,{rear},{port}} = {P_{a}{Sd}\frac{\left( \frac{Zd}{{Sd}^{2}} \right){{Zpv}}}{\left( \frac{Zd}{{Sd}^{2}} \right){{{Zpv} + {Zpp} + {Zpr}}}}}} & (11)\end{matrix}$

Impedance element Zpv is the air-volume acoustic compliance calculatedfrom (13) with V denoting the cavity volume $\begin{matrix}{{Zpv} = \frac{\rho\quad c^{2}}{{j\omega}\quad V}} & (12)\end{matrix}$

Furthermore, Zpp is the port acoustic impedance, consisting of a dampingelement and a mass element in series connection, e.g. calculated for anarrow slit which typically is introduced for purposes of elevateddamping rates (14). The narrow slit impedance may be estimated from itslength 1 (parallel to the sound propagation direction), its width a(orthogonal to sound propagation and the least distance between twoopposite planes in the slit) and the slit height b the constant ηdenotes the air viscosity (approximately 18.3 10⁻⁶ Ns/m).$\begin{matrix}{{Zpp} = {\frac{12\eta\quad l}{a^{3}b} + {{j\omega}\frac{6}{5}\frac{\rho\quad l}{ab}}}} & (13)\end{matrix}$

The resulting force acting on the sensor diaphragm in system where therear side diaphragm pressure signal has passed the port-volumeacoustical filter system then becomes the sum of the contribution fromeach side.F _(amb,port) =F _(d,closed) +F _(d,rear,port)  (14)

A transducer of Type III and with the dimensions and weight given abovewill perform as shown in FIG. 10. Here the variation has been given (topto bottom) in ah, ad, mh, and cd, the latter being the compliance of thediaphragm. Solid lines indicate the nominal value, the dashed linesindicate double the nominal respective values and the dotted linesindicate half the nominal respective values.

Besides the simple rear side opening and the port-volume filter openingdescribed above, there exist a wide variety of interesting principlesfor guiding/filtering the ambient noise signal in its attack on thediaphragm rear side. Examples could include an acoustic horn, e.g.having the larger area end pointing against the surroundings and thenarrow area end connecting to the sensor diaphragm. Also an acousticwaveguide consisting of multiple coupled ports and cavities, oralternatively passive diaphragms (as known from slave bass loudspeakersystems) could prove interesting in the optimization of the transducerimmunity towards ambient noise.

As mentioned above, regarding type II, the acoustic resistance means mayusefully be found in the group comprising felt and non-woven fibrousmaterials, however, they may have to be very compact. Alternatively, theport in the case of Type III, when formed as a slit in the housing, mayhave an appreciable length and a correspondingly narrow width. This hasthe particular advantage that accidental partial closure will notdisturb the function to an appreciable degree. The provision of anon-wettable surface in the slit precludes any trapping of water. Inpractice, this may be obtained by a PTFE insert with a laser-cut slit.

Similar considerations to those mentioned above concerning Type I andType II will conclude that also forType III there may be obtained adistinct advantage by keeping the Sd/Sh range within the limitsaccording to the invention, and the fact that more paramaters areavailable for variation in Type III enable the contribution of the Sd/Shto be tailored to a specific desired frequency response expressed as aS/N ratio. The value 1/20 functions well in this environment.

The foregoing description of the specific embodiments will so fullyreveal the general nature of the present invention that others skilledin the art can, by applying current knowledge, readily modify or adaptfor various applications such specific embodiments without undueexperimentation and without departing from the generic concept, andtherefore, such adaptations and modifications should and are intended tobe comprehended within the meaning and range of equivalents of thedisclosed embodiments. It is to be understood that the phraseology orterminology employed herein is for the purpose of description and not oflimitation. The means, materials, and steps for carrying out variousdisclosed functions may take a variety of forms without departing fromthe invention.

Thus, the expressions “means to . . . ” and “means for . . . ”, or anymethod step language, as may be found in the specification above and/orin the claims below, followed by a functional statement, are intended todefine and cover whatever structural, physical, chemical, or electricalelement or structure, or whatever method step, which may now or in thefuture exist which carries out the recited functions, whether or notprecisely equivalent to the embodiment or embodiments disclosed in thespecification above, i.e., other means or steps for carrying out thesame function can be used; and it is intended that such expressions begiven their broadest interpretation.

1-15. (canceled)
 16. A transducer for bioacoustic signals, comprising: atransducer element having a front side and a rear side, the front sidebeing adapted for establishing intimate contact with a surface of a bodypart receiving direct interior sound from the body, said transducerelement being mounted in a housing subject to airborne noise, and havinga surface surrounding the front side of said transducer element, saidtransducer element and said surrounding surface being in intimatecontact with the surface of said body part during use, wherein thetransducer element has an effective area that is less than 50% of thearea of the surrounding surface of the housing, and wherein the rearside of the transducer element is loaded by acoustical network meanswhich are in communication with the surrounding air, said loadingcreating an extinguishing relationship between airborne noise signalsinfluencing the front and rear sides of the transducer element,respectively.
 17. A transducer according to claim 16, wherein theeffective area of the transducer element fulfills the area ratio0.50≧ad/ah≧0.001, where ad is the effective area and ah is the area ofthe surrounding surface.
 18. A transducer according to claim 16, whereinthe effective area of the transducer element fulfills the area ratio0.20≧ad/ah≧0.05, where ad is the effective area and ah is the area ofthe surrounding surface.
 19. A transducer according to claim 16, whereinthe transducer element is a compound diaphragm which produces anelectrical output when subjected to bending.
 20. A transducer accordingto claim 16, wherein the transducer element is a compound diaphragmwhich produces an electrical output when subjected to differentialstretching of a front side with respect to a rear side of the diaphragm.21. A transducer according to claim 16, wherein the acoustical networkmeans comprises a cavity in the housing which is indirectly influencedby airborne noise.
 22. A transducer according to claim 16, wherein thetransducer element is a compound diaphragm and wherein the acousticalnetwork means comprises a cylindrical conduit having essentially thesame diameter as the diaphragm.
 23. A transducer according to claim 16,wherein the acoustical network means comprises a cavity and at least oneport in the housing.
 24. A transducer according to claim 23, wherein theport is formed by a narrow slit.
 25. A transducer according to claim 24,wherein the slit is made in a material that is not wetted by water. 26.A transducer according to claim 16, wherein an elastic material capableof transmitting mechanical vibration is provided in sealing relationshipwith respect to the diaphragm in a manner sealing the diaphragm relativeto the surface of a body part in use.
 27. A transducer according toclaim 16, wherein the acoustical network means comprises a dampingmaterial.
 28. A transducer according to claim 27, wherein the transducerelement is a compound diaphragm, wherein the acoustical network meanscomprises a cylindrical conduit having essentially the same diameter asthe diaphragm and wherein the cylindrical conduit is provided with adamping material.
 29. A transducer according to claim 27, wherein theacoustical network means comprises a damping material, and wherein thedamping material is used as a resistive element in a port.
 30. Atransducer according to claim 27, wherein the damping material haswater-repellent qualities.